In position controllers, friction characteristics that targets to be controlled (hereinafter referred to as controlled targets) have in machinery systems vary greatly depending on temperature and across the ages; therefore, even if, taking into consideration the friction characteristics so that overshoot never happens, a gain most suitable for the controlled target is set, due to the varying of the friction characteristics, the overshoot may sometimes happen when the friction increases, or, on the contrarily, even if the overshoot does not happen when the friction decreases, it sometimes takes a longer time before the position has been decided. Therefore, in a machine in which the friction varies, in order to prevent overshoot in response to such friction characteristic variations, technique has been needed to automatically adjust the control gain.
An example of conventional technique is disclosed in Japanese Patent Publication No. 3,463,355. According to this conventional technique, a plant is driven by applying thereto a speed-command signal for identifying viscous friction, and by constructing a speed PI-control system in a servo amplifier, “PI-speed-control-system command torque” that is a signal outputted from the system, and “equivalent PI-speed-control-system command torque” in which the signal outputted from the system is passed through a primary delay filter are observed. Because the steady value of the difference signal between the “PI-speed-control-system command torque” and the “equivalent PI-speed-control-system command torque” converges on a value depending on a viscous friction coefficient, the viscous friction coefficient is identified using the value and given parameters.
Next, in a speed control system, a value obtained by multiplying an actual speed in the plant by the identified value of the viscous friction coefficient is set as viscous friction compensation torque, and then the torque is added to the command torque. As a result, the method of the conventional technique prevents, with this viscous-friction compensation torque compensating the friction torque, overshoot due to friction.
[Patent Document 1]
Japanese Patent Publication No. 3,463,355.